Connection structure for multi-core fiber and optical-fiber-bundle structure, connection structure for multi-core fibers, method for exciting rare-earth-doped multi-core fibers, and multi-core-optical-fiber amplifier

ABSTRACT

An optical-fiber-bundle structure is connected to one end of a multi-core fiber. The multi-core fiber has a tapered section formed therein. The outside diameter of the multi-core fiber and the core pitch thereof decrease in the tapered section. It is possible for the multi-core fiber to have an increasing core pitch on the connection-side thereof which connects to the optical-fiber-bundle structure; hence, it is possible to use an easy-to-use large-diameter optical fiber as the optical fiber to be provided in the optical-fiber-bundle structure. When connecting another multi-core fiber to the other end of the multi-core fiber, it is possible to match the outer diameters thereof; hence, when fusion splicing to one another, it is unlikely for a positional shift of the cores to occur.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a connection structure for a multi-core fiberhaving a plurality of cores and an optical-fiber-bundle structure, and aconnection structure for multi-core fibers.

BACKGROUND OF THE INVENTION

The transmission capacity of single-core optical fibers used in thepresent situations is approaching its limit due to the recent rapidtraffic increase in optical communications. As a means for furtherexpanding the communication capability, a multi-core fiber, which is afiber having a plurality of cores formed therein, has been proposed.Using a multi-core fiber reduces the cost for laying optical fibers andallows the communication capability to expand.

When a multi-core fiber is used as a transmission line, fan-out, whichseparates each core of the multi-core fiber into single-core fibers, isrequired. To connect such a multi-core fiber and single-core fibers,there is a method in which optical fibers are in a close-packedarrangement, forming a bundle structure that is to be connected with themulti-core fiber (Patent Document 1).

Also, there is another method in which a plurality of single-core fibersis bundled to be drawn, integrated, and then connected to a multi-corefiber (Patent Document 2).

Also, there is another method in which the diameter of one end of amulti-core fiber is enlarged and the light is introduced into the coreof the multi-core fiber (Patent Document 3).

RELATED ART Patent Documents

-   [Patent Document 1] WO2012121320A1-   [Patent Document 2] Japanese Patent No. 3415449-   [Patent Document 3] Japanese Unexamined Patent Application    Publication No. 2001-145562

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Since manufacturing of an optical-fiber-bundle structure is easy, such amethod proposed in Patent Document 1, in which single-core fibers havinga cladding diameter equivalent to a core pitch of a multi-core fiber arebundled, has high practicability. Particularly, a multi-core fiber usedfor long distance transmission requires a large core pitch to preventdeterioration by crosstalk. In this case, it is possible to usesingle-core fibers having diameters that correspond to the core pitch ofthe multi-core fiber.

On the other hand, for short-distance transmission such as in office, itis possible to decrease the core pitch of a multi-core fiber. Therefore,in view of capacity and handling properties (allowable bending radius),it is preferable to reduce the diameter as much as possible. Also, arare-earth doped multi-core fiber used as an optical amplifier (erbiumdoped multi-core fiber for example) is used in short distances.Particularly, it is preferable for a cladding-pumped rare-earth-dopedmulti-core fiber to reduce the cladding diameter and the core pitch inview of pump efficiency. This is because, if the cladding diameter of arare-earth-doped multi-core fiber is large, the optical densitydecreases to the pump light of the cladding region so to lower the pumpefficiency.

Thus, if the core pitch of a multi-core fiber is decreased (50 μm orless for example), it is necessary to reduce the diameters of thecorresponding single-core fibers. However, such small-diameter opticalfibers have low rigidity and poor handling properties. Also, since thecladding thickness is thin, sealing of light into the core becomes leakyand the transmission loss is large. Furthermore, the thin claddingthickness is likely to cause micro-bend loss against external force tooccur. Therefore, there is a limit in reducing the diameter ofsingle-core fibers that form an optical-fiber-bundle structure.

However, as in Patent Document 2, with a method in which a plurality ofoptical fibers is bundled and then heated and melted to reduce thediameters, the external form of the bundle structure is deformed so thatdeformation of the cores or un-uniformity of core positions may occur.Therefore, connection loss with a multi-core fiber may become large.

In the method described in Patent Document 3, sections in which the corepitch is wide are made by changing drawing conditions and the like.However, this method has a problem that it is not possible to cut andconnect at any desired places after drawing.

The present invention was achieved in view of such problems. Its objectis to provide a connection method for a multi-core fiber and anoptical-fiber-bundle structure with less transmission loss and the like.

Means for Solving Problems

To achieve the above object, a first invention is a connection structurefor a multi-core fiber and an optical-fiber-bundle structure wherein theoptical-fiber-bundle structure comprises a plurality of signal lightoptical fibers that transmit signal lights that are arranged atpredetermined intervals, the multi-core fiber comprises cores, which areoptically connected to the signal light optical fibers that transmitsignal lights at a first end on the connection side that is connected tothe optical-fiber-bundle structure, and a cladding covering the coresand having a refractive index lower than that of the cores. Themulti-core fiber is a tapered fiber having a tapered section in which anouter diameter thereof varies. The outer diameter and the core pitch ofthe multi-core fiber at the first end are wider than the outer diameterand the core pitch of the multi-core fiber at a second end, which is onthe opposite side of the connection part of the optical-fiber-bundlestructure.

The multi-core fiber may be further connected to a second multi-corefiber and the core of the second multi-core fiber may be opticallyconnected with the core of the multi-core fiber.

The second end of the multi-core fiber approximately matches with theouter diameter of an opposing end of the second multi-core fiber and theouter diameter of at least one of the second end of the multi-core fiberand the opposing end of the second multi-core fiber may be reduced byetching without any changes in the core pitch

The optical-fiber-bundle structure may further comprise a pump lightoptical fiber that transmits pump light, and the second multi-core fibermay be a rare-earth-doped multi-core fiber. In this case, the pump lightoptical fiber that transmits pump light is a multi-mode fiber and thecore of the pump light optical fiber that transmits pump light may beconnected to the cladding of the first end of the multi-core fiber.

The pump light optical fiber that transmits pump light may be arrangedat the center of the optical-fiber-bundle structure and the signal lightoptical fibers that transmit signal lights may be arranged on the outercircumference of the pump light optical fiber that transmits pump light.

The diameter of the pump light optical fiber that transmits pump lightmay be larger than the diameter of the signal light optical fiber thattransmits signal light.

According to the first invention, since the multi-core fiber is atapered fiber in which the outer diameter and the core pitch vary, thecore pitch on the side of connection part that is connected with theoptical-fiber-bundle structure can be increased. Therefore, it ispossible to increase the outer diameter of the signal light opticalfiber that transmits signal light which is connected with the multi-corefiber. Therefore, the handling properties of the signal light opticalfibers that transmit signal lights are excellent and the opticaltransmission loss can be suppressed.

Also, by further connecting with a second multi-core fiber, a desiredoptical connection between a multi-core fiber and anoptical-fiber-bundle structure can be established with a multi-corefiber for pitch conversion.

Also, matching the outer diameters at the connection part between themulti-core fiber for pitch conversion and the second multi-core fibercan suppress position shifting of the cores at the time of fusionsplicing. On this occasion, it is possible to match both outer diameterseasily by reducing the diameter of at least one end of the multi-corefiber for pitch conversion and the other multi-core fiber by etching.

Also, if the optical-fiber-bundle structure includes the pump lightoptical fiber that transmits pump light and the second multi-core fiberis a rare-earth doped multi-core fiber, the optical-fiber-bundlestructure and the rare-earth doped multi-core fiber can be connected bythe multi-core fiber for pitch conversion. On this occasion, it is notnecessary to excessively decrease each fiber-diameter of theoptical-fiber-bundle structure, and it is also unnecessary toexcessively increase the diameter of the rare-earth doped multi-corefiber.

Also, arranging the pump light optical fiber that transmits pump lightat the center of the optical-fiber-bundle structure makes it possiblefor the pump light to be introduced into the center of the rare-earthdoped multi-core fiber. On this occasion, since it is possible to use alarger pump light optical fiber that transmits pump light by making theouter diameter of the pump light optical fiber that transmits pump lightlarger than the outer diameter of the signal light optical fiber thattransmits signal light, large output of pump light can be introducedinto the rare-earth doped multi-core fiber.

A second invention is a connection structure for multi-core fibers,wherein a first multi-core fiber having a plurality of cores isconnected with a second multi-core fiber having an outer diameter thatis different from that of the first multi-core fiber and a plurality ofcores that are optically connected with the cores of the firstmulti-core fiber. At least one of the first multi-core fiber and thesecond multi-core fiber has an outer diameter varying section, and theouter diameters of the end part of the first multi-core fiber on theconnection-side and the end part of the second multi-core fiber on theconnection part are almost equivalent.

The outer diameter varying section is formed by drawing a multi-corefiber and may include a section in which the core pitch at thecross-section varies along with the outer diameter.

The outer diameter varying section is formed by etching a multi-corefiber and may include a section in which the core pitch at thecross-section does not vary and only the outer diameter varies.

According to the second invention, it is possible to efficiently connectmulti-core fibers having different outer diameters.

On this occasion, forming the outer diameter varying section by drawingat least one of the multi-core fibers can vary the core pitch. Also, ifthe outer diameter varying section is formed by etching, only the outerdiameter can be varied without varying the core pitch.

A third invention is a method for exciting a multi-core fiber,comprising a step of connecting an optical-fiber-bundle structure, amulti-core fiber, and a rare-earth doped multi-core fiber. Theoptical-fiber-bundle structure comprises a plurality of signal lightoptical fibers that transmit signal lights, which are arranged at apredetermined pitch, and a pump light optical fiber that transmits pumplight. The multi-core fiber comprises cores, which are opticallyconnected to the signal light optical fibers that transmit signal lightsat a first end thereof on the connection-side that is connected with theoptical-fiber-bundle structure, and a cladding having a refractive indexlower than that of the cores and covering the cores. The rare-earthdoped multi-core fiber comprises cores, which are optically connected tothe cores of the multi-core fiber at a second end that is on theopposite side of the connection part of the multicore fiber and theoptical-fiber-bundle structure, and a cladding having a refractive indexlower than that of the cores and covering the cores. The multi-corefiber is a tapered fiber having a tapered section in which the outerdiameter thereof varies and the outer diameter and the core pitch at thefirst end of the multi-core fiber are wider than the outer diameter andthe core pitch at the second end. The pump light optical fiber thattransmits pump light is a multi-mode fiber and the core of the pumplight optical fiber that transmits pump light is connected to thecladding of the first end of the multi-core fiber.

A fourth invention is a multi-core amplifier, comprising anoptical-fiber-bundle structure, a multi-core fiber, and a rare-earthdoped multi-core fiber. The optical-fiber-bundle structure comprises aplurality of signal light optical fibers that transmit signal lights,which are arranged at a predetermined pitch, and a pump light opticalfiber that transmits pump light. The multi-core fiber comprises cores,which are optically connected to the signal light optical fibers thattransmit signal lights at a first end thereof on the connection-sidethat is connected with the optical-fiber-bundle structure, and acladding having a refractive index lower than that of the cores andcovering the cores. The rare-earth doped multi-core fiber comprisescores, which are optically connected to the cores of the multi-corefiber at a second end that is on the opposite side of the connectionpart of the multicore fiber and the optical-fiber-bundle structure, anda cladding having a refractive index lower than that of the cores andcovering the cores. The multi-core fiber is a tapered fiber having atapered section in which the outer diameter thereof varies and the outerdiameter and the core pitch at the first end of the multi-core fiber arewider than the outer diameter and the core pitch at the second end. Thepump light optical fiber that transmits pump light is a multi-mode fiberand the core of the pump light optical fiber that transmits pump lightis connected to the cladding of the first end of the multi-core fiber.

According to the third and the fourth inventions, it is possible toefficiently amplify signal light in multi-core fibers.

Effects of the Invention

The present invention can provide a connection method for a multi-corefiber and an optical-fiber-bundle structure with less transmission lossand the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a connection structure for an optical fiber 1.

FIG. 2( a) shows a cross-sectional view of A-A line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 2( b) shows a cross-sectional view of B-B line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 3( a) shows a cross-sectional view of C-C line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 3( b) shows a cross-sectional view of D-D line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 4( a) shows a cross-sectional view of E-E line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 4( b) shows a cross-sectional view of F-F line of the connectionstructure for an optical fiber 1 in FIG. 1.

FIG. 5( a) to FIG. 5( c) show a forming process of a tapered section.

FIG. 6( a) to FIG. 6( c) show a connection process of the taperedsection and a capillary 21 c.

FIG. 7 shows a connection structure for an optical fiber 1 a.

FIG. 8( a) shows a cross-sectional view of G-G line of the connectionstructure for an optical fiber 1 a in FIG. 7.

FIG. 8( b) shows a cross-sectional view of H-H line of the connectionstructure for an optical fiber 1 a in FIG. 7.

FIG. 9 shows a connection structure for an optical fiber 1 b.

FIG. 10 shows a connection structure for an optical fiber 1 c.

FIG. 11( a) shows a cross-sectional view of I-I line of the connectionstructure for an optical fiber 1 c in FIG. 10.

FIG. 11( b) shows a cross-sectional view of J-J line of the connectionstructure for an optical fiber 1 c in FIG. 10.

FIG. 12( a) shows a cross-sectional view of M-M line of the connectionstructure for an optical fiber 1 c in FIG. 10.

FIG. 12( b) shows a cross-sectional view of N-N line of the connectionstructure for an optical fiber 1 c in FIG. 10.

FIG. 13 shows a connection structure for an optical fiber 1 d.

FIG. 14( a) shows a cross-sectional view of O-O line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 14( b) shows a cross-sectional view of P-P line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 15( a) shows a cross-sectional view of Q-Q line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 15( b) shows a cross-sectional view of R-R line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 16( a) shows a cross-sectional view of S-S line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 16( b) shows a cross-sectional view of T-T line of the connectionstructure for an optical fiber 1 d in FIG. 13.

FIG. 17( a) shows a cross-sectional view of a bundle structure 5 c.

FIG. 17( b) shows a cross-sectional view of a bundle structure 5 d.

DESCRIPTION OF EMBODIMENTS First Embodiment

Hereinafter, a connection structure for an optical fiber 1 will bedescribed with reference to the accompanying drawings. FIG. 1 shows theconnection structure for an optical fiber 1, FIG. 2( a) is across-sectional view of A-A line in FIG. 1, FIG. 2( b) is across-sectional view of B-B line in FIG. 1, FIG. 3( a) is across-sectional view of C-C line in FIG. 1, FIG. 3( b) is across-sectional view of D-D line in FIG. 1, FIG. 4( a) is across-sectional view of E-E line in FIG. 1, and FIG. 4( b) is across-sectional view of F-F line in FIG. 1.

The connection structure for an optical fiber 1 is a connectionstructure of a bundle structure 5 and multi-core fibers 3 a, 3 b. Themulti-core fibers 3 a, 3 b and an optical fiber 7 a are made of, forexample, quartz glass.

The bundle structure 5 comprises optical fibers 7 a that are bundled.The optical fibers 7 a are signal light optical fibers that transmitsignal lights. The optical fibers 7 a are preferably single-mode opticalfibers that are appropriate for long distance transmission and, in thiscase, the signal light is, for example, single mode at 1550 nm band.

As shown in FIG. 2( a), the bundle structure 5 has a plurality ofoptical fibers 7 a assembled together in a close-packed arrangement.That is, one optical fiber 7 a is arranged at the center with sixoptical fibers 7 a arranged on the circumference thereof. Therefore, allcores 15 a of each of the optical fibers 7 a are arranged at equalintervals. Hereinafter, unless specified, each of the optical fibers 7 ahas a fixed outer diameter from one end of the bundle structure 5 to theoutside of the other end of the bundle structure 5. The outer diametersof optical fibers 7 a may be bundled after being adjusted by chemicaletching using hydrofluoric acid and the like, drawing by heating andmelting, or the like.

Although the example in the drawing shows an example having the sevenoptical fibers 7 a, the present invention is not limited thereto. It isonly necessary that the number of optical fibers 7 a is six or more andthe close-packed arrangement is possible. For example, the outerdiameter of the optical fiber 7 a in the center may be larger than theouter diameter of the optical fiber 7 a on the outer circumference side.In this case, it is necessary that the optical fiber in the center is incontact with all the optical fibers on the outer circumference and theadjacent optical fibers on the outer circumference are arranged to be incontact with each other without any gaps. Using the dense bundlestructure as above facilitates the optical connection with a pluralityof cores of the multi-core fiber 3 a.

The optical fibers 7 a are arranged inside a capillary 21 b. The opticalfibers 7 a and the capillary 21 b are bonded with resin 19. Thecapillary 21 b is made of a material having a refractive index lowerthan the refractive index of the material forming the resin 19. Theresin 19 is made of a material having a refractive index lower than therefractive index of the material forming a cladding 17 a of the opticalfiber 7 a.

The bundle structure 5 is connected to one end of the multi-core fiber 3a. The end face of the multi-core fiber 3 a and the end face of thebundle structure 5 are both grounded and arranged to be facing eachother. On this occasion, each of the cores 15 a and the cores 22 a faceeach other at the positions in which each of the cores 15 a and thecores 22 a are optically connected. The multi-core fiber 3 a and thebundle structure 5 are connected by bonding or fusion splicing. Also,the capillary 21 a, 21 b are bonded. The multi-core fiber 3 a is, forexample, a single-mode fiber, and has a tapered section 27 which is anouter-diameter varying section formed on a part thereof.

As shown in FIG. 2( b), the end part of the multi-core fiber 3 a on theconnection-side connected to the bundle structure 5 (the one end) isinserted into the capillary 21 a and fixed with the resin 19. Thecapillary 21 a is made of a material having a refractive index lowerthan the refractive index of the material forming the resin 19. Theresin 19 is made of a material having a refractive index lower than therefractive index of the material of a cladding 23 a of the multi-corefiber 3 a.

Also, the outer diameter of the cladding 23 a is approximatelyequivalent to or slightly larger than a circle circumscribing theoptical fibers 7 a that configure the bundle structure 5. The cores 22 aof the multi-core fiber 3 a are arranged in the positions and at thepitch that correspond to the cores 15 a of the bundle structure 5 (theoptical fibers 7 a), and the cores 15 a and the cores 22 a are opticallyconnected. That is, the seven cores 22 a in total are arranged at thecenter of the multi-core fiber 3 a and at each vertex of a hexagonsurrounding it. In this case, all the distances between the core 22 a atthe center and each of the six surrounding cores 22 a are the same.Also, for the six cores 22 a, the distances between each of the adjacentcores 22 a are the same.

For example, the core pitch of the cores 22 a of the multi-core fiber 3a is 50 μm, and, in this case, the cladding diameter of the opticalfiber 7 a forming the bundle structure 5 is 50 μm.

As shown in FIG. 3( a), a resin-coating layer 25 is provided on theouter circumference of the part of the multi-core fiber 3 a that isexposed outside the capillary 21 a. The resin forming the resin-coatinglayer 25 is made of a material having a refractive index lower than therefractive index of the material forming the cladding 23 a of themulti-core fiber 3 a.

In FIG. 3( a), the pitch of the cores 22 a is P1 and the outer diameterof the cladding 23 a is D1. P1 is almost equal to the outer diameter ofthe optical fiber 7 a.

As shown in FIG. 3( b), the outer diameter and the core pitch of themulti-core fiber 3 a decrease in the tapered section 27. That is, themulti-core fiber 3 a is reduced in the diameter at the vicinity of theend which is opposite to the connection-side of the bundle structure 5of the multi-core fiber 3 a (the other end). Therefore, in the taperedsection, a core pitch P2 is smaller than P1, and a cladding diameter D2is smaller than D1. Since the thickness of the resin-coating layer 25increases in the tapered section 27, the overall outer diameterincluding the resin-coating layer 25 remains substantially constant.

As shown in FIG. 4( a), the other end of the multi-core fiber 3 a isinserted into the capillary 21 c and fixed with the resin 19. Thecapillary 21 c is made of a material having a refractive index lowerthan the refractive index of the material forming the resin 19. Theresin 19 is made of a material having a refractive index lower than therefractive index of the material forming a cladding 23 a of themulti-core fiber 3 a. A core pitch P3 at the other end of the multi-corefiber 3 a is further smaller than P2, and an outer diameter D3 isfurther smaller than D2.

Although the illustrated example shows the tapered section 27 beinginserted into the capillary 21 c at the middle of the tapered section,the entire tapered section 27 may be inserted into the capillary 21 c,or only the tip side of the tapered section 27 (reduced diameter side)may be inserted into the capillary 21 c. Also, although the illustratedexample shows that the inner diameter of the capillary 21 c is setbetween the maximum outer diameter part (D1) and the minimum outerdiameter part (D3) of the multi-core fiber 3 a (the outer diameters atthe both ends of the tapered section 27 respectively), the innerdiameter of the capillary 21 c may be larger than D1.

The multi-core fiber 3 a is, for example, fusion spliced with anothermulti-core fiber 3 a. The multi-core fiber 3 b is, for example, asingle-mode fiber. As shown in FIG. 4( b), the end part of themulti-core fiber 3 b on the connection-side with the multi-core fiber 3a is inserted into a capillary 21 d and fixed with the resin 19. Thecapillary 21 d is made of a material having a refractive index lowerthan the refractive index of the material forming the resin 19. Theresin 19 is made of a material having a refractive index lower than therefractive index of the material forming a cladding 23 b of themulti-core fiber 3 b.

The core pitch P3 of the other end of the multi-core fiber 3 a isequivalent to a core pitch P4 of the multi-core fiber 3 b. For example,the core pitch P4 is 50 μm or less. If P4 is 45 μm, for example, thecore pitch at the one end of the multi-core fiber 3 a is 50 μm and thecore pitch at the other end can be 45 μm.

An outer diameter of the cladding 23 b (D4) is approximately equivalentto the outer diameter of the cladding 23 a (D3) or slightly larger thanthe outer diameter of the cladding 23 a (D3). That is, D3≦D4. Also, thecores 22 b of the multi-core fiber 3 b are arranged in the positionscorresponding to the cores 22 a of the multi-core fiber 3 a. Therefore,the cores 22 a and the cores 22 b are optically connected.

Thus, the core pitch can be changed by using the multi-core fiber 3 a,which is a tapered fiber. That is, the multi-core fiber 3 a can be usedas a pitch-changing multi-core fiber. Therefore, it is possible to makethe core pitch and the outer diameter of the bundle structure 5 largerthan the core pitch and the outer diameter of the multi-core fiber 3 b.

Next, a manufacturing method for a tapered fiber will be described. Asshown in FIG. 5( a), the multi-core fiber 3 a having the resin-coatinglayer 25 on the outer circumference thereof is used. First, as shown inFIG. 5( b), a predetermined range of the resin-coating layer 25 from theend part of the multi-core fiber 3 a is peeled off. Next, as shown inFIG. 5( c), the part in which the resin-coating layer 25 is peeled offis heated and melted to be drawn. The tapered section 27 is then formedin this way.

Next, as shown in FIG. 6( a), the outer circumference of the multi-corefiber 3 a, including the tapered section 27, is coated by resin to formthe resin-coating layer 25 once again. As above, a tapered fiber isformed. Furthermore, as shown in FIG. 6( b), a part of the resin-coatinglayer 25 from the end part of the multi-core fiber 3 a to the middle ofthe tapered section 27 is peeled off. In this state, as shown in FIG. 6(c), the end part of the multi-core fiber 3 a in which the resin-coatinglayer 25 is peeled off is inserted into the capillary 21 c and fixedwith the resin 19. Furthermore, grinding the end face of the capillary21 c allows a connection with the multi-core fiber 3 b.

As described above, according to the present embodiment, it is possibleto easily connect the bundle structure having a relatively wide corepitch with the multi-core fiber 3 b having a relatively narrow corepitch.

The multi-core fiber 3 a has different mode-field diameters at the oneend and the other end. Also, in this case, it is preferable toapproximately match the mode-field diameter of the one end of themulti-core fiber 3 a and the mode-field diameter of the optical fiber 7a. Also, it is preferable to approximately match the mode-field diameterof the other end of the multi-core fiber 3 a and the mode-field diameterof the end part of the multi-core fiber 3 b. This can solve the increasein the transmission loss caused by the mismatching of mode-fielddiameters at each connection part.

Although the multi-core fiber 3 b and the bundle structure 5 areconnected through the medium of multi-core fiber 3 a in this embodiment,the multi-core fiber 3 a and the multi-core fiber 3 b may be integrated.That is, the object of the present invention can be achieved byconnecting the bundle structure 5 with a multi-core fiber with varyingcore pitch.

Second Embodiment

Next, a second embodiment will be described. FIG. 7 shows a connectingstructure for an optical fiber 1 a, and FIG. 8( a) is a cross-sectionalview of G-G line in FIG. 7 and FIG. 8( b) is a cross-sectional view ofH-H line in FIG. 7. For the embodiments below, the same notations willbe used for the components performing the same functions as in theconnection structure for an optical fiber 1, and redundant explanationswill be omitted.

The connection structure for an optical fiber 1 a has a configurationthat is almost the same as that of the connection structure for anoptical fiber 1 except that a diameter-reduction section 29, which is anouter diameter varying section, is formed at the tip part of amulti-core fiber 3 a. The diameter-reduction section 29 can be formed bychemical etching using fluoric acid and the like.

The core pitch and the cladding diameter of the multi-core fiber 3 a arechanged by the tapered section 27. As shown in FIG. 8( a), the corepitch is P5 and the cladding diameter is D5 after diameter reduction.Also, as shown in FIG. 8( b), the core pitch at the diameter-reductionsection 29 is P6 and the cladding diameter is D6. In this case, the corepitch P5 and P6 are equivalent, but the cladding diameter D6 is smallerthan the cladding diameter D5. That is, only the outer diameter becomessmaller at the diameter-reduction section 29 while the core pitchremains the same.

Here, it is preferable that the cladding diameter at the end part of themulti-core fiber 3 a is smaller than the cladding diameter of themulti-core fiber 3 b. For example, if D5>D6, only the outer diameter ofthe end part of the multi-core fiber 3 a is reduced so that D5<D6. It isnot likely for the position shifting between cores at the time of fusionsplicing to occur if the cladding diameter of the end part of themulti-core fiber 3 a matches the cladding diameter of the multi-corefiber 3 b. That is, the diameter-reduction section 29 can match thecladding diameter of the end part of the multi-core fiber 3 a with thecladding diameter of the multi-core fiber 3 b.

The diameter-reduction section 29 may be formed at the end part of themulti-core fiber 3 b as in a connection structure for an optical fiber 1b shown in FIG. 9. Also, although drawing is omitted, thediameter-reduction section 29 may be formed at the both end parts of themulti-core fiber 3 a, 3 b. Thus, using the diameter-reduction section 29allows multi-core fibers with different outer diameters to easilyconnect with each other. For example, the tapered section 27 is formedby drawing process to match the core pitch, and with the matched corepitch, the diameter-reduction section 29 may be formed by etching at theend part of the multi-core fiber on the side with a larger claddingdiameter.

Thus, according to the second embodiment, it is possible to easilyconnect multi-core fibers having different outer diameters with eachother. On this occasion, by matching the outer diameters, it is unlikelyfor deformation or misalignment of axis of the cores to occur at thetime of fusion splicing. Although the bundle structure 5 is connectedwith the multi-core fibers 3 a, 3 b in the illustrated example, the useof the diameter-reduction section 29 and the tapered section 27 makes iteffective for connecting multi-core fibers without the bundle structure5.

Third Embodiment

Next, a third embodiment will be described. FIG. 10 shows a connectingstructure for an optical fiber 1 c, and FIG. 11( a) is a cross-sectionalview of I-I line in FIG. 10, FIG. 11( b) is a cross-sectional view ofJ-J line in FIG. 10, FIG. 12( a) is a cross-sectional view of M-M linein FIG. 10, and FIG. 12( b) is a cross-sectional view of N-N line inFIG. 10.

The connection structure for an optical fiber 1 c has a configurationthat is almost the same as that of the connection structure for anoptical fiber 1 except that the bundle structure 5 a is used therein. Inaddition to the optical fibers 7 a, the bundle structure 5 a furtherincludes an optical fiber 7 b bundled. The optical fiber 7 b is a pumplight optical fiber that transmits pump light introduction. Hereinafter,unless specified, each optical fiber 7 b has a fixed diameter from anend face of the bundle structure 5 to the outside of the bundlestructure 5 on the side of the other end.

A light source 8 for pump light is connected to the optical fiber 7 b.The light source 8 is, for example, a multi-mode pump light emitter witha wavelength of 980 nm. Although the optical fiber 7 b may be asingle-mode or multi-mode optical fiber, a multi-mode optical fiber ispreferable because the core diameter can be increased for high power. Inthis case, the pump light is, for example, multi-mode light in thewavelength band of 980 nm.

In the bundle structure 5 a, the optical fibers 7 a are in aclose-packed arrangement, and the optical fiber 7 b is further arrangedon the outer circumference thereof. The outer diameter of the opticalfiber 7 b is larger than the outer diameter of the optical fiber 7 a.The optical fiber 7 b comprises a core 15 b and a cladding 17 b. Thediameter of the core 15 b of the optical fiber 7 b is larger than thediameter of the core 15 a of the optical fiber 7 a. In this way, lightwith more power can be introduced.

The bundle structure 5 a is spliced with one end part of the multi-corefiber 3 a. The outer diameter of the cladding 23 a of the multi-corefiber 3 a is approximately equivalent to or slightly larger than acircle circumscribing the optical fibers 7 a, 7 b that configure thebundle structure 5 a. The core 15 b of the optical fiber 7 b correspondsto the position of the cladding 23 a of the multi-core fiber 3 a. Thatis, as shown in FIG. 11( b), a pump light introduction section 31 ispositioned in the cladding 23 a.

If the core pitch of the multi-core fiber 3 a is 50 μm and the claddingthickness (the distance from the outermost core to the cladding surface)is 75 μm, the outer diameter of the optical fiber 7 b can be increasedup to 62 μm. Furthermore, if the core 15 b of the optical fiber 7 b ismade to be included in the cladding 23 a of the multi-core fiber 3 a,then the outer diameter of the optical fiber 7 b can be increased up to73 μm.

A tapered fiber may also be used as the optical fiber 7 b. For example,a step index multi-mode optical fiber with a core diameter of 105 μm andcladding diameter of 125 μm can be used as the optical fiber 7 b beforetaper processing and the outer diameter at the connection part with themulti-core fiber 3 a may be drawn processed by heating and melting orthe like to correspond to the multi-core fiber 3 a.

The pump light is confined in the cladding 23 a by the resin 19. Sincethe resin-coating layer 25 is provided on the part of the multi-corefiber 3 a that is exposed out of the capillary 21 a, the pump light isconfined in the cladding 23 a.

FIG. 12( a) is an end part of the multi-core fiber 3 a and FIG. 12( b)is an end part of the multi-core fiber 3 b. In the present embodiment,the multi-core fiber 3 b is a rare-earth doped multi-core fiber. Therare-earth doped multi-core fiber is, for example, an erbium-dopedmulti-core fiber (EDF).

The claddings 23 a, 23 b of the multi-core fibers 3 a, 3 b are opticallyconnected. Therefore, the pump light introduced into the cladding 23 aof the multi-core fiber 3 a is transmitted through the cladding 23 a ofthe multi-core fiber 3 a and then introduced into the cladding 23 b ofthe multi-core fiber 3 b. Introducing the pump light into the multi-corefiber 3 b can excite an rare-earth element included in the core 22 b,which is doped by a rare-earth material such as erbium, of themulti-core fiber 3 b and amplify the signal light inside the core 22 b.That is, the pump light can bring the energy level of the erbium ions inthe core 22 b to an excited state. By introducing the signal light intothe core 22 b in this state, stimulated emission of the excited erbiumions occurs and the intensity of the signal light is amplified.

The diameter-reduction section 29 may also be formed in the presentembodiment on an end part of either the multi-core fiber 3 a or 3 b.

As described above, according to the third embodiment, the multi-corefiber 3 b can perform optical amplification. On this occasion, since itis possible to decrease the cladding diameter of the multi-core fiber 3b, pump efficiency is excellent. Thus, an efficientmulti-core-optical-fiber amplifier can be formed according to thepresent embodiment.

Also, the core pitch of the bundle structure 5 a that is connected withthe multi-core fiber 3 b can be wider than the core pitch of themulti-core fiber 3 b. Therefore, the outer diameter of the optical fiber7 a can be larger than the core pitch of the multi-core fiber 3 b.Therefore, a connection structure for an optical fiber that is excellentin handling properties and has small optical transmission loss can beobtained.

Fourth Embodiment

Next, a fourth embodiment will be described. FIG. 13 shows a connectingstructure for an optical fiber 1 d, and FIG. 14( a) is a cross-sectionalview of O-O line in FIG. 13, FIG. 14( b) is a cross-sectional view ofP-P line in FIG. 13, FIG. 15( a) is a cross-sectional view of Q-Q linein FIG. 13, FIG. 15( b) is a cross-sectional view of R-R line in FIG.13, FIG. 16( a) is a cross-sectional view of S-S line in FIG. 13, andFIG. 16( b) is a cross-sectional view of T-T line in FIG. 13.

The connection structure for an optical fiber 1 d has a configurationthat is almost the same as that of the connection structure for anoptical fiber 1 c except that a bundle structure 5 b is used therein. Asshown in FIG. 14( a), the bundle structure 5 b has an optical fiber 7 barranged at the center. The optical fiber 7 b is connected with thelight source 8, which is omitted in the drawing. Also, the claddingdiameter of the optical fiber 7 b is larger than the cladding diameterof the optical fiber 7 a. The optical fibers 7 a are arrangedclose-packed on the outer circumference of the optical fiber 7 b.

In the example shown in the drawing, ten optical fibers 7 a are arrangedclose-packed on the outer circumference of the optical fiber 7 b. In thepresent invention, as shown in FIG. 14( a), the close-packed arrangementis an arrangement in which the optical fibers 7 a are positioned atpredetermined intervals so to be in contact with each other.

The bundle structure 5 b is fusion spliced with the one end of themulti-core fiber 3 a. As shown in FIG. 14( b), the outer diameter of thecladding 23 a of the multi-core fiber 3 a is approximately equivalent toor slightly larger than the circle circumscribing the optical fibers 7 athat configure the bundle structure 5 b. The core 15 b of the opticalfiber 7 b corresponds to the approximately center position of thecladding 23 a of the multi-core fiber 3 a. That is, the pump lightintroduction section 31 is positioned at the approximately center of thecladding 23 a. The pump light introduced into the cladding 23 a isconfined inside the cladding 23 a by the resin 19.

As shown in FIG. 15( a) and FIG. 15( b), the resin-coating layer 25 isprovided on the outer circumference of the multi-core fiber 3 a that isexposed out of the capillary 21 a. Thus, since the part of themulti-core fiber 3 a that is exposed out of the capillary 21 a isprovided with the resin-coating layer 25 on the outer circumference, thepump light is transmitted being confined in the cladding 23 a.

FIG. 16( a) shows an end part of the multi-core fiber 3 a and FIG. 16(b) shows an end part of the multi-core fiber 3 b. Also, in the presentembodiment, the multi-core fiber 3 b is a rare-earth doped multi-corefiber.

The claddings 23 a, 23 b of the multi-core fibers 3 a, 3 b are opticallyconnected. Therefore, the pump light is introduced into the cladding 23a of the multi-core fiber 3 a. Therefore, by introducing the signallight into the core 22 b, stimulated emission of the excited erbium ionsoccurs and the intensity of the signal light is amplified.

The diameter-reduction section 29 may also be formed in the presentembodiment on an end part of either of the multi-core fiber 3 a and 3 b.

According to the fourth embodiment, the same effects as the thirdembodiment can be obtained. Also, since the pump light is introduced tothe approximately center of the cladding 23 a, it is possible tointroduce the pump light almost uniformly to each of the cores 22 b.

A bundle structure 5 c as shown in FIG. 17( a) may also be used asanother bundle structure having the optical fiber 7 b arranged at thecenter thereof. The bundle structure 5 c includes the optical fibers 7a, 7 b having approximately the same diameters. Therefore, six signallight introduction fibers are arranged on the outer circumference of thepump light introduction fiber at the center.

Also, a bundle structure 5 d as shown in FIG. 17( b) may be used. Thebundle structure 5 d includes eight optical fibers 7 a arrangedclose-packed on the outer circumference of the optical fiber 7 b. Thus,in the present invention, it is required that the outer diameter of theoptical fiber 7 b is equal to or larger than the diameter of the opticalfiber 7 a and the optical fibers 7 a are arranged close-packed withoutany gaps.

Although the embodiments of the present invention have been describedreferring to the attached drawings, the technical scope of the presentinvention is not limited to the embodiments described above. It isobvious that persons skilled in the art can think out various examplesof changes or modifications within the scope of the technical ideadisclosed in the claims, and it will be understood that they naturallybelong to the technical scope of the present invention.

DESCRIPTION OF NOTATIONS

-   1, 1 a, 1 b, 1 c, 1 d . . . connection structure for optical fiber-   3 a, 3 b . . . multi-core fiber-   5, 5 a, 5 b, 5 c, 5 d . . . optical-fiber-bundle structure-   7 a, 7 b . . . optical fiber-   8 . . . light source-   15 a, 15 b . . . core-   17 a, 17 b . . . cladding-   19 . . . resin-   21 a, 21 b, 21 c, 21 d . . . capillary-   22 a, 22 b . . . core-   23 a, 23 b . . . cladding-   25 . . . resin-coating layer-   27 . . . tapered section-   29 . . . diameter-reduction section-   31 . . . pump light introduction section

What is claimed is:
 1. A connection structure for a multi-core fiber andan optical-fiber-bundle structure wherein: the optical-fiber-bundlestructure comprises a plurality of signal light optical fibers thattransmit signal lights that are arranged at predetermined intervals; themulti-core fiber comprises cores, which are optically connected to thesignal light optical fibers that transmit signal lights at a first endthereof on the connection side that is connected with theoptical-fiber-bundle structure, and a cladding having a refractive indexlower than that of the cores and covering the cores; and the multi-corefiber is a tapered fiber having a tapered section in which the outerdiameter thereof varies, the outer diameter and the core pitch at thefirst end of the multi-core fiber being wider than the outer diameterand the core pitch at a second end of the multi-core fiber, which is onthe opposite side of the connection part that is connected with theoptical-fiber-bundle structure.
 2. The connection structure for amulti-core fiber and an optical-fiber-bundle structure according toclaim 1, wherein: the multi-core fiber is further connected to a secondmulti-core fiber; and the core of the second multi-core fiber isoptically connected with the core of the multi-core fiber.
 3. Theconnection structure for a multi-core fiber and an optical-fiber-bundlestructure according to claim 2, wherein: the second end of themulti-core fiber approximately matches with the outer diameter of anopposing end of the second multi-core fiber; and the outer diameter ofat least one of the second end of the multi-core fiber and the opposingend of the second multi-core fiber is reduced by etching without anychanges in the core pitch
 4. The connection structure for a multi-corefiber and an optical-fiber-bundle structure according to claim 2,wherein: the optical-fiber-bundle structure further comprises a pumplight optical fiber that transmits pump light; and the second multi-corefiber is a rare-earth-doped multi-core fiber.
 5. The connectionstructure for a multi-core fiber and an optical-fiber-bundle structureaccording to claim 4, wherein: the pump light optical fiber thattransmits pump light is a multi-mode fiber and the core of the pumplight optical fiber that transmits pump light is connected to thecladding of the first end of the multi-core fiber.
 6. The connectionstructure for a multi-core fiber and an optical-fiber-bundle structureaccording to claim 4, wherein: the pump light optical fiber thattransmits pump light is arranged at the center of theoptical-fiber-bundle structure and the signal light optical fibers thattransmit signal lights are arranged on the outer circumference of thepump light optical fiber that transmits pump light.
 7. The connectionstructure for a multi-core fiber and an optical-fiber-bundle structureaccording to claim 6, wherein: the diameter of the pump light opticalfiber that transmits pump light is larger than the diameter of thesignal light optical fiber that transmits signal light that transmitsignal light.
 8. A connection structure for multi-core fibers, wherein:a first multi-core fiber having a plurality of cores is connected with asecond multi-core fiber having an outer diameter that is different fromthat of the first multi-core fiber and a plurality of cores that areoptically connected with the cores of the first multi-core fiber; atleast one of the first multi-core fiber and the second multi-core fiberhas an outer diameter varying section; and the outer diameters of theend part of the first multi-core fiber on the connection-side and theend part of the second multi-core fiber on the connection part arealmost equivalent.
 9. The connection structure for multi-core fibersaccording to claim 8, wherein: the outer diameter varying section isformed by drawing a multi-core fiber and includes a section in which thecore pitch at the cross-section thereof varies along with the outerdiameter.
 10. The connection structure for multi-core fibers accordingto claim 8, wherein: the outer diameter varying section is formed byetching a multi-core fiber and includes a section in which the corepitch at the cross-section thereof does not vary and only the outerdiameter varies.
 11. A method for exciting a rare-earth doped multi-corefiber, comprising a step of: connecting an optical-fiber-bundlestructure comprising a plurality of signal light optical fibers thattransmit signal lights, which are arranged at a predetermined pitch, anda pump light optical fiber that transmits pump light; a multi-core fibercomprising cores, which are optically connected to the signal lightoptical fibers that transmit signal lights at a first end thereof on theconnection-side that is connected with the optical-fiber-bundlestructure, and a cladding having a refractive index lower than that ofthe cores and covering the cores; and an rare-earth doped multi-corefiber comprising cores and claddings that are doped with a rare-earthmaterial, the cores and the claddings being optically connected to thecores and the claddings of the multi-core fiber respectively at a secondend that is on the opposite side of the connection part of the multicorefiber and the optical-fiber-bundle structure, wherein the multi-corefiber is a tapered fiber having a tapered section in which the outerdiameter thereof varies and the outer diameter and the core pitch at thefirst end of the multi-core fiber are wider than the outer diameter andthe core pitch at the second end; the pump light optical fiber thattransmits pump light is a multi-mode fiber and the core of the pumplight optical fiber that transmits pump light is connected to thecladding of the first end of the multi-core fiber; pump light isintroduced into the pump light optical fiber that transmits pump light;and the pump light is transmitted through the cladding of the multi-corefiber, introduced into the cladding of the rare-earth doped multi-corefiber, and excites a rare-earth material included in the cores of therare-earth doped multi-core fiber.
 12. A multi-core-optical-fiberamplifier, comprising: an optical-fiber-bundle structure comprising aplurality of signal light optical fibers that transmit signal lights,which are arranged at a predetermined pitch, and a pump light opticalfiber that transmits pump light; a multi-core fiber comprising cores,which are optically connected to the signal light optical fibers thattransmit signal lights at a first end thereof on the connection-sidethat is connected with the optical-fiber-bundle structure, and acladding having a refractive index lower than that of the cores andcovering the cores; an rare-earth doped multi-core fiber comprisingcores and claddings that are doped with a rare-earth material, the coresand the claddings being optically connected to the cores and thecladdings of the multi-core fiber respectively at a second end that ison the opposite side of the connection part of the multicore fiber andthe optical-fiber-bundle structure; and a light source that introducespump light into the core of the pump light optical fiber that transmitspump light, wherein the multi-core fiber is a tapered fiber having atapered section in which the outer diameter thereof varies and the outerdiameter and the core pitch at the first end of the multi-core fiber arewider than the outer diameter and the core pitch at the second end; andthe pump light optical fiber that transmits pump light is a multi-modefiber and the core of the pump light optical fiber that transmits pumplight is connected to the cladding of the first end of the multi-corefiber.